1. Packet filter: Packet filtering inspects each packet passing through the network and accepts or rejects it based on user-defined rules. 2. Circuit-level firewall:  Applies security mechanisms when a TCP or UDP connection is established. Once the connection has been made, packets can flow between the hosts without further checking. 3. Application gateway:  Applies security mechanisms to specific applications, such as FTP and Telnet servers. 4. Proxy server:  Intercepts all messages entering and leaving the network acting as an intermediary between clients and servers. The proxy server hides the true network addresses